This invention relates to electromechanical actuation mechanisms, and more particularly relates to mechanically-bistable actuation mechanisms.
Mechanically-bistable mechanisms are important for enabling a wide range of electromechanical systems. Such bistable mechanisms, having two positions of displacement that are both mechanically stable, are particularly well-suited for two-position actuation applications, such as relays, valves, threshold switches, memory cells, clasps, grippers, and other applications. The natural actuation stiffness of bistable mechanisms inherently are well-suited to binary applications that require two conditions such as open/closed, up/down, etc. An important advantage provided for such applications is the inherent hysteresis in the bistable force-displacement characteristic, which enhances the robustness and immunity of bistable mechanisms to external disturbances.
Bistable mechanisms have been proposed for applications in all size regimes, including that of microelectromechanical systems (MEMs), which typically are fabricated on the scale of millimeters or microns. Many actuator applications require the size and precision of MEMs, and such are becoming increasingly common for actuation components as well as systems. There have been proposed a range of MEMs structures that meet component and system design criteria by providing bistable actuation displacement. For example, there has been proposed a buckled-beam mechanism that incorporates latches for enabling actuation between two stable positions. Similarly, there has been proposed a hinged, multi-segment mechanism that relies on hinging action to enable stable actuation between two positions. While operational, these two designs suffer in that they require very particular latch or hinge configurations and corresponding actions; these requirements tend to severely limit the design parameters of an associated actuation system.
There further has been proposed the design of MEMs structures having bistability imposed not by a mechanical configuration but by a selected condition of axial preloading implemented as, e.g., compressive stress. FIG. 1A is a schematic representation, shown in side view, of such a structure, here a single suspended beam 10, having a clamped boundary 12 and a sliding boundary 13. The beam is characterized by a prescribed level of residual compressive stress imposed during fabrication of the beam. The view of FIG. 1A illustrates an initial, un-actuated, uncurved and stable condition of the beam. FIG. 1B is a schematic side view of the single beam when displaced, following a displacement profile that is dictated by the residual compressive stress in the beam, here schematically represented as an axial preload 14 applied at one end. Positions 11a and 11c are two symmetric and stable positions. Position 11b is an unstable equilibrium position, which has an S-shape. Thus the application of an axial preload by the inclusion of residual stress results in a displacement profile that includes two stable positions of the suspended beam structure.
Similarly, FIG. 2A provides a schematic representation, shown also in side view, of a double beam actuator 20 in its initial condition, and having imposed on the structure some prescribed level of residual compressive stress. The double beam actuator 20 is composed of two straight beams 22 and 24 clamped at both boundaries 25 and 26, with a central clamp member 23 connecting the two straight beams. Note that the quiescent state of the beams is a straight, uncurved geometry; the beams are provided as-fabricated in this straight, uncurved geometry. FIG. 2B is a schematic side view of the double beam structure when displaced; the compressive residual stress imposed on the structure being represented by an axial preload 27 applied at one end. Displacement positions 21a and 21c are two symmetric, stable positions while position 21b is an unstable equilibrium position. Thus, like the single beam described above, bistable actuation can be imposed on this double-beam structure by the inclusion of residual compressive stress in the beams of the structure.
It is found that in practice it can be extremely difficult to implement the residual compressive stress required to enable bistable actuation of the single and double beam structures just described. More specifically, it is difficult to implement the compressive stress preloading in a precise, reproducible manner that is sufficiently robust to be deployed in large scale manufacturing operations such as commercial microfabrication. Indeed, it can be particularly difficult, if not impossible, to attain precise preloading in micro-scale structures such as those employed in MEMs actuation applications. One common technique for mechanically preloading a MEMs structure is the provision of residual compressive stress from fabrication processes, e.g., material coating, doping, or structural feature provision. But such techniques are found to be hard to control precisely and repeatably, and over time, the precision required of fabrication equipment to reliably provide a specified stress is difficult to control.
Beyond the limitations of such fabrication techniques, it is found that accurate and reliable provision of a pre-specified residual stress in or preloading of a structure is difficult by any technique and for any size regime. As a result, commercial deployment of actuators having bistable operation imposed by material stress conditions has been severely limited. Complicated and often constraining mechanical components, such as the latches and hinges described above, are thus typically required for producing bistable actuation. But many applications cannot accommodate the additional complexity required of such componentry. The deployment of bistable actuation systems has thus been significantly limited, overly complicated, or excessively unreliable.